1. Technical Field
This invention relates to the illumination of a telephone dial, and more particularly, to the illumination of such dial through use of fiber optic light guides.
2. Description of the Prior Art
Incandescent lights have been the primary source for illuminating telephone dials for dialing in low light or darkened conditions for a number of years. These lights require commercial power which is first stepped down by a line transformer and then distributed to each telephone set at a premises over an additional wire pair often available in standard telephone wiring. Such additional wires are not now available in many premises and the line transformer is both inconvenient and costly. Thus in recent years light-emitting diodes (LEDs) considered to be more desirable for illuminating telephone dials have been replacing the incandescent lights. These LEDs are powered by the current provided over the telephone line to the telephone set from a switching station such as a central office.
One of the perceptions of product quality in a telephone set is the uniformity of illumination across the face of the telephone dial if illumination for the dial is provided. In order to maintain this uniform illumination, a number of LEDs are typically employed, because most telephone sets receive their operating power from the switching station, which may be some distance away from the set, the available power is often limited.
A telephone dial employing LEDs is disclosed in U.S. Pat. No. 4,349,705. In this patent, a push-button dial assembly comprises a plurality of layers in a sandwich form. One of the layers is a planar plate made from a transparent plastic material. The plate includes four cylindrical recesses, each receiving a LED. The LEDs are energized when the telephone set is in its off hook (active) state and light travels through the light guide plate to translucent pushbuttons located on the keypad of the telephone set. Although this technique provides generally good results, the limited available power cannot be used exclusively by the LEDs, but rather must be shared with electrical circuits used for communications in the telephone set.
With the advent of improved cordless telephone sets in recent years, high quality wireless communications have come to be expected by the average consumer. Included in that high quality is convenience and ease of operation of these cordless telephone sets, including dial illumination inasmuch as the handset unit of these telephone sets is portable and may be easily transported to areas having low light or darkened conditions. A limitation on the illumination of the dial of a cordless telephone set not present in a corded telephone set is that the handset unit of the cordless telephone set requires a battery for providing operating power while it is operated remote from an associated base unit. Thus power for operating the handset unit is not only limited like that for the corded telephone set, but also exhaustible.
To insure the longest possible period of operation for a handset unit before having to return this unit to its associated base unit where it is normally recharged, many techniques have been devised for minimizing the current drain on the handset unit's battery. One such technique is to turn the handset unit completely off until or unless an incoming call is expected. Another technique, disclosed in U.S. Pat. No. 4,731,814 issued on Mar. 15, 1988 to W. R. Becker et al., reduces current drain by reducing the on-time state of most of the electronics in the handset unit when this unit is operated in a standby state, i.e., awaiting receipt of a call or other user initiated action. Thus in a cordless telephone handset unit, it is desirable to provide illumination for a telephone dial with minimum power in order to maintain the longest possible period of operation between battery recharges. While achieving this minimum power operation, it is also desirable that uniformity of illumination across the face of the telephone dial be maintained and/or improved.